Subject: What is the function of the acid in the polymerisation of kevlar?

I think I understand the overall reaction but I am puzzled by the acid. My 
initial thoughts are that the acid will protonate the carbonyl oxygen atom 
of the carboxylic acid group to produce a diol carbocation. This can then 
be attacked by the unbonded electron pair of the nitrogen in the amine 
group of the benzyl diamine component. I just dont understand why 
nucleophilic attack of the carbonyl carbon atom cannot take place without 
protonation leading to the carbocation formation. Is the amine group not 
nucleophilic enough as it is? Or am I not understanding the function of 
the acid?

Re: What is the function of the acid in the polymerisation of kevlar?